Projection display technologies may sometimes require complex optical geometries to fit around obstacles to a linear optical path. An example is the optical geometry used on some helmet-mounted display (HMD) devices in order to “bend” image-bearing light around the helmet. These optical geometries must direct image-bearing light from an image source to a projection screen or mirror on a helmet. The optics may incorporate a display device that has a restricted Etendue, due to the constraints imposed on the optical geometry by the shape of the helmet. The Etendue, in these circumstances, may be too small to fully illuminate the exit pupil of the projection optics. This is one example of a scenario in which a restricted Etendue can arise.
The Etendue of an optical system can be defined as follows. From the point of view of a light source, it is the area of the source times the solid angle subtended by the entrance pupil of the system as seen from the source. From the system point of view, the Etendue is the area of the entrance pupil times the solid angle the light source subtends as seen from the pupil. A perfect optical system produces an image with the same Etendue as the source. The term has a general meaning of “geometrical extent”. Other names for this property are “acceptance”, “throughput”, “light-grasp”, “collecting power”, and “optical extent”, as are well known in the art.